Electroporation refers to the phenomena of rearranging the structure of the membrane or membranes of cells to introduce or modify porosity across the membrane film, thereby creating a mechanism for transport between the extra-cellular and intracellular fluids, caused by application of an electric field. (Zimmerman U, Electromanipulation of Cells, CRC Press, Boca Raton Fla., 1996, herein incorporated by reference).
Pulsed electric fields have long been under investigation for causing many different biological effects. Yet, in spite of decades of research, there is an incomplete understanding of the interaction of electromagnetic fields within biological cells and tissues. Investigations of pulsed electric fields and microwave radiation aimed at achieving cell effects such as electroporation have historically utilized relatively long pulse lengths, such as pulses greater than 1 μsecond, and microwave radiation approaching the thermal-heating regime. Studies of the interactions of RF and microwave electromagnetic fields on biological systems have been limited by the use of these long pulse lengths, or continuous wave radiation, which reduces the coupling of high electric fields into the interior of the cell.
Aqueous pores, typically about 1 nm in diameter, have creation rates typically on the order of microseconds, and possibly shorter with rapidly pulsed fields. Depending on the process for pore formation, resealing of a pore may take much longer (Weaver J C, Chizmadzhev Y A, Theory of Electroporation: A Review, Bioelectrochemistry and Bioenergetics, v41, 1996, pp. 135-160; Bier M, Hammer S M, Canaday D J, Lee R C, Kinetics of Sealing for Transient Electropores in Isolated Mammalian Skeletal Muscle Cells, Bioelectromagnetics, v20, 1999, pp. 194-201, herein incorporated by reference). Typical field strengths required for electroporation vary between hundreds of volts/cm to kilovolts/cm, depending on the duration of the field. The external field increases the transmembrane potential from about 80 mV to a much larger value, facilitating porosity. It has been consistently shown that once the transmembrane potential reaches or exceeds about the one volt threshold, pores form, resulting in membrane permeabilization, molecular uptake, or lysis from osmosis. There is limited understanding of the membrane dynamics during pore formation. Although modeling captures some linear and even nonlinear aspects of electroporation, the model itself must use variables empirically derived from gathered data, and are qualitative, because of the present limited understanding of membrane physics (Schoenbach K H, Perterkin F E, Alden R W, Beebe S J, The Effect of Pulsed Electric Fields on Biological Cells: Experiments and Applications, IEEE Transactions on Plasma Science, v25, 1997, pp. 284-292, herein incorporated by reference).